TN295 



No. 9169 



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IC9169 



Bureau of Mines Information Circular/1987 



Coal Combustion in a Ventilated Tunnel 



By Margaret R. Egan 




UNITED STATES DEPARTMENT OF THE INTERIOR 




c i£ t S]UA&* i L 



-TvU-vU* ) 



Information Circular; 91 69 




Coal Combustion in a Ventilated Tunnel 



By Margaret R. Egan 



UNITED STATES DEPARTMENT OF THE INTERIOR 
Donald Paul Hodel, Secretary 

BUREAU OF MINES 

David S. Brown, Acting Director 




^\\l^ 



\]H 



n 



b< 



■\ 



l&1 



Library of Congress Cataloging in Publication Data: 



Egan, Margaret R. 

Coal combustion in a ventilated tunnel. 

(Bureau of Mines information circular; 9169) 

Bibliography: p. 12. 

Supt. of Docs, no.: I 28.27: 9169. 

1. Coal mines and mining — Fires and fire prevention. I. Title. II. Series: Information 
circular (United States. Bureau of Mines); 9169. 



TN295.U4 [TN315] 



622 s 



[622'.8] 87-600251 



CONTENTS 

Page 

Abstract 1 

Introduction 2 

Properties of coal 2 

Fire tunne 1 2 

Instrumentation 3 

Thermocouples 3 

Flow probes and pressure transducers 3 

Gas monitors 4 

Smoke monitors 4 

Fuel-consumption monitor 5 

Typical test procedure 5 

Calculations 5 

Product generation rates 5 

Combustion yields 6 

Heat-release rates 6 

Production constants 6 

Smoke particle diameters 7 

Coal combustion results and discussion 7 

Gas concentrations and heat production 8 

Smoke characteristics 9 

Combustion yields 9 

Production constants 10 

Fuel comparison results and discussion 10 

References 12 

Appendix. — Symbols used in this report 13 

ILLUSTRATIONS 

1. Schematic of intermediate-scale tunnel and data-acquisition system 3 

2. Results of a typical coal combustion test 8 

TABLES 

1. Ventilation, ignition, and mass loss for coal experiments 7 

2. Gas concentrations, generation rates, heat-release rates, and heats of 

combustion 9 

3. Smoke characteristics for coal 9 

4. Mean particle sizes and smoke obscuration 10 

5. Combustion yields for coal 10 

6. Production constants for coal 10 

7. Ignition source and ventilation rates for the three fuels tested 10 

8. Gas, heat, and smoke concentrations 11 

9. Normalized gas and smoke concentrations 11 

10. Particle size and smoke obscuration 11 

11. Production constants 11 





UNIT OF MEASURE ABBREVIATIONS 


USED IN THIS 


REPORT 


Btu/lb 


British thermal unit 
per pound 




kW 
m 


kilowatt 
meter 


cm 


centimeter 




mg/m 3 


milligram per cubic 


cm 


cubic centimeter 






meter 


g 


gram 




min 


minute 


g/cm 3 


gram per cubic centimeter 


Mm 


micrometer 


g/(m 3 *ppm) 


gram per cubic meter 
part per million 


times 


m 3 /s 


cubic meter per 
second 


g/g 


gram per gram 




p/cm 3 


particle per cubic 
centimeter 


g/kJ 


gram per kilojoule 




p/g 


particle per gram 


g/s 


gram per second 




p/kJ 


particle per 


kg 


kilogram 






kilojoule 


kJ 


kilojoule 




ppm 


part per million 


1 kJ/g 


kilojoule per gram 




V 


volt 



COAL COMBUSTION IN A VENTILATED TUNNEL 

By Margaret R. Egan' 



ABSTRACT 

The Bureau of Mines experimentally burned Pittsburgh Seam coal and 
other combustible materials found in mines in order to obtain a better 
knowledge of their emission products. These experiments were conducted 
in the Bureau's intermediate-scale fire tunnel, which simulates environ- 
mental conditions in underground mines. Smoke characteristics, gas 
concentrations, mass loss, and ventilation were measured. From these 
values, heat-release rates, particle sizes, obscuration rates, combus- 
tion yields, and production constants were calculated. 

This information was sought as part of a comprehensive study of 
combustible materials that will ultimately advance the design of more 
efficient fire detection and suppression systems. The coal combustion 
measurements presented in this report, together with previous analyses 
of wood and transformer fluid fires form a data base by which future 
studies of other mine combustibles can be compared. 



1 Chemist, Pittsburgh Research Center, Bureau of Mines, Pittsburgh, PA. 



INTRODUCTION 



The Bureau of Mines conducts research 
to ensure that mines are safe and healthy 
places to work. Exceptional circum- 
stances, such as an underground fire, 
pose dangers affecting escape and rescue. 
In any fire, each burning material con- 
tributes hazardous combustion products. 
Each component of the smoke poses unique 
dangers to escape and rescue, and in 
combination, the components become even 
more dangerous. The life-threatening 
hazards of smoke and toxic gas produced 
by burning mine materials may be carried 
throughout the mine by the ventila- 
tion system. Therefore, the Bureau has 
been investigating fire characteristics 
in ventilated passageways. Once the 
hazardous products of combustible ma- 
terials are determined, more efficient 



detection and rescue equipment can be 
designed. 

This report supplements previous Bureau 
studies Q. - .^) 2 of other combustible ma- 
terials burned in a simulated mine envi- 
ronment. The intermediate-scale fire 
tunnel used for the current studies was 
shown to successfully predict full-scale 
fire conditions ( 3_) . The tunnel was 
instrumented with gas and smoke analyzers 
as well as a data collection system that 
can perform calculations and plot data as 
the input information is received. 

The objectives of this study were to 
analyze the gas production and smoke 
characteristics of burning coal and to 
compare these findings with data for the 
other combustible materials previously 
studied. 



PROPERTIES OF COAL 



Coal is a familiar substance, but it 
has no fixed chemical formula. It was 
formed from decomposing plant material 
that was subjected to increased tempera- 
ture and pressure for a prolonged period 
of time. The composition of the coal is, 
therefore, dependent upon the composition 
of the original plant material. However, 
all coals have carbon, hydrogen, and oxy- 
gen as major elements, with sulfur and 
nitrogen as minor elements. Bituminous 
coal from the Pittsburgh Seam was used 
for all experiments. Its ultimate analy- 
sis revealed the following: 78% C, 5.3% 
H, 8.2% 0, 5.6% ash, 1.6% N, and 1.3% S 



(4_). The proximate analysis showed the 
following: moisture content, 1.7%; ash 
content, 5.6%; volatile matter, 38.8%; 
and fixed carbon 53.9% ( 4_) . The heating 
value was 13,947 Btu/lb (4). 

As coal is heated, it decomposes into 
an ash residue and gaseous volatiles. 
Among these gases are CO2, CO, nitrogen, 
steam, oxides of sulfur and nitrogen, and 
hydrocarbons such as methane. The two 
most important gases for calculating heat 
release are CO2 and CO. These were 
measured continuously throughout these 
experiments. 



FIRE TUNNEL 



The coal fires were conducted in the 
Bureau's intermediate-scale fire tunnel. 
The tunnel and its data-acquisition sys- 
tem are shown in figure 1. The tunnel 
measures 0. 8 m wide by 0.8 m high by 10 m 
long and is divided into several sec- 
tions. The first horizontal section is 
1.5 m long and can be lifted to allow en- 
trance for the placement of the coal. It 
begins with an air-intake cylinder that 
measures 0.25 m long by 0.3 m in diameter 
and gradually enlarges until it matches 



the tunnel dimensions at the hinged area. 
Next is the fire zone, where the coal 
pile and a gas burner are located. The 
fire zone and the remaining horizontal 
section are lined with fire brick and 
instrumented with thermocouples, flow 
probes, and sampling ports. The dif- 
fusing grid begins the vertical section 



2 Under lined numbers in parentheses re- 
fer to items in the list of references 
preceding the appendix. 



10m 



12m 



/0.61m -diam duct 



1.22m 



Fire zone — i 
Air • __ — ^— *- 



0.8m- square duct 



intake 




Sd 



HI 



^p^ — i — * 
Coal pile A, 



^ 



1 



Air 



exhaust 



Manually Ventilation 

^U\ adjustable . J an , . 

Fy or if ir.P nlntP 



Load eel 



ice plate 
Diffusing grid 



-0.305m- diam 
entrance duct 
( hinged and movable) 



DECNET- 



TEOM 



CNM 



CO meter 



CO2 meter 



Pressure transducers 
MM 



48" 
channel 
data- 
acqui- 
sition 
system 



3\ 

- detector 



-Load cell 

-Digital input 

for CNM 

range 



PDP 
II /44 



Control 
terminal 



VAX 

11/780 



Printer 



CALCOMP 
plotter 



VAX 
terminal 



"••• 28 •••• 
thermocouples 



Pressure transducer (flow probe 
Differential pressure transducer 
3X detector 
Thermocouples 
Sampling ports 



KEY 

CALCOMP California computer products 

CNM Condensation nuclei monitor 

DECNET Digital equipment networking 

PDP Programmed data processor 

TEOM Tapered-element oscillating microbalance 

VAX Virtual address extension 



FIGURE 1. — Schematic of intermediate-scale tunnel (top) and data-acquisition system (bottom). 



of the tunnel. Located in this section 
is an orifice plate that can be manually 
adjusted to attain the desired airflow. 

INSTRUMENTATION 



The final section is horizontal and ends 
at an exterior exhaust fan. 



All instruments were periodically 
cleaned and calibrated according to manu- 
facturers ' instructions for the quantity 
of smoke and the amount of use each had 
received. 

THERMOCOUPLES 

Thermocouple arrays were located 1.57, 
2.36, 3.15, 4.72, 6.30, and 7.87 m from 
the gas burner. Additional thermocouples 
were located on the air-intake cone and 



at the exhaust. In all, a total of 28 
thermocouples were used to measure the 
temperature distributions resulting from 
the fires. Their locations are shown in 
figure 1. 

FLOW PROBES AND PRESSURE TRANSDUCERS 

The velocity data were acquired using a 
bidirectional flow probe (_5) in con- 
junction with a pressure transducer. 
The airflow produced by the exhaust 



ventilation was detected by the flow 
probe and converted to a linear electri- 
cal signal by the pressure transducer. 
This signal was then scanned and recorded 
by the data collection system. The loca- 
tions of all of the flow probes are shown 
in figure 1. The flow probe used to 
obtain the velocity measurements was the 
one centered in the air-intake cylinder. 

The stated accuracy of the flow probe 
is ±7%. The pressure transducer added a 
maximum error of ±5.3%. Assuming the 
error to be accumulative, the maximum 
velocity error for one data point was 
estimated to be ±12.3%. Averaging over 
10 data points improves the accuracy by 
the square root of 10, resulting in a 
total estimated error of ±3.9% for the 
average data presented. 

Additional velocity readings were made 
with a vane-type anemometer. This was 
done before each experiment to insure 
that the air-intake velocity was approxi- 
mately the same for all tests. 

GAS MONITORS 

The CO analyzer used measures accurate- 
ly within 1% of full range or ±5 ppm. 
The C0 2 analyzer measures accurately 
within 1% of full range or ±250 ppm. 
These analyzers were calibrated at the 
beginning of each experiment. In addi- 
tion, the concentrations of the span 
gases were checked at the beginning of 
each series of experiments. 

The hydrogen cyanide (HCN) detector's 
accuracy is stated as ±1% of the reading. 
Its calibration was checked before this 
series of experiments began and rechecked 
after its completion. 

SMOKE MONITORS 

The number concentration (N Q ) was ob- 
tained with a Condensation Nuclei Monitor 
(CNM), manufactured by Environment One 
Corp., Schenectady, NY. 3 This monitor 
uses a cloud chamber to measure the 
concentration of submicrometer airborne 

-^Reference to specific products does 
not imply endorsement by the Bureau of 
Mines. 



particles (p). The particulate cloud 
attenuates a light beam which ultimately 
produces a measurable electrical signal. 
The accuracy is stated as ±20% of a point 
above 30% of scale within the linear 
ranges from 3,000 to 300,000 p/cm 3 . 
Therefore, in these experiments, the cal- 
culated error could have been ±18,000 
p/cm 3 . 

To reduce the particulate count to 
within the range of the CNM, a 10% dilu- 
tion of the smoke was necessary. Two 
flow meters, with a stated accuracy of 
±2%, were used. One measured the flow of 
the sample, and the other measured fil- 
tered room air. The dilution error was 
calculated to be from -15.2 to +22%. 
Over 10 data points, this error was 
reduced to ±7%. Adding this error to the 
already stated error of the CNM increases 
the total error to ±27%, making this the 
least accurate of all the instruments. 

The mass concentration (M ) was ob- 
tained by a tapered-element oscillating 
microbalance (TEOM) developed by Rup- 
precht & Patashnick Co. , Inc. , Voorhees- 
ville, NY. (6) It measures the mass 
directly by depositing the particles on a 
filter attached to an oscillating tapered 
element. The oscillating frequency of 
the tapered element decreases as the 
deposited mass increases. The apparatus 
is capable of measuring the particulate 
concentration with a better than 5% ac- 
curacy at the level used. According to 
the manufacturer, the filter collects at 
least 50% of all particles with a volume 
mean diameter of 0.05 ym, with increasing 
collection efficiency as the diameter 
increases. Actual data obtained by the 
Bureau using particles of volume mean 
diameter equal to 0.048 ym indicated a 
collection efficiency closer to 90%. 

Since the diameter of average mass is 
calculated from the mass and number con- 
centrations, its accuracy was dependent 
upon the precision of the TEOM and CNM. 
Considering the error band of the mass 
and number concentration produced by the 
coal combustion experiments, the diameter 
of average mass could vary by 14%. 

A three -wave length light-transmission 
technique developed by the Bureau (_7) was 
also used to measure smoke concentration 



and obscuration. White light was trans- 
mitted through a smoke cloud to the 
detector. The beam was split into three 
parts, and each passed through an inter- 
ference filter centered at wavelengths of 
either 0.45, 0.63, or 1.00 um. Each 
photodiode output was amplified and re- 
corded as a linear electric signal. 



FUEL-CONSUMPTION MONITOR 

The weight-loss data were obtained by a 
strain-gauge conditioner in conjunction 
with a load cell that has a range up to 
22.68 kg. Their combined accuracy is 
stated as 0.05% of full scale or ±11.3 g. 



TYPICAL TEST PROCEDURE 



Approximately 18 kg of coal was broken 
into pieces measuring 125 cm or less and 
placed in a 59- by 68-cm stainless steel 
pan. The pan was supported by a shaft 
that extended through the tunnel floor 
and was mounted on the load cell so that 
continuous weight loss could be recorded. 

Prior to each experiment, background 
readings were obtained after the coal was 
loaded and the exhaust fan started. All 
instruments were continuously scanned and 
recorded throughout the experiment. 

The coal was ignited by three strip 
heaters that were equidistantly imbedded 
in the coal approximately 2.5 cm beneath 
the surface. The timing of all experi- 
ments began when the strip heaters were 
turned on. The power was gradually 
increased for three 5-min intervals until 
it reached a maximum of 90 V. After 20 
min at that level, the power was turned 



off and the coal continued to burn. 
After sufficient data were obtained, the 
fire was extinguished and the experiment 
was concluded. 

The 21 experiments were divided into 
three phases. In the first phase (tests 
1 through 6), the strip heaters were of 
unequal length. The two end ones were 52 
cm long, and the center one was 37 cm 
long. The ventilation rate averaged 0.24 
m /s. In the second phase (tests 7 
through 8), the ventilation rate was 
raised to an average speed of 0.44 m /s. 
In the third phase (tests 9 through 21), 
the ventilation rate averaged 0.23 m /s , 
and three 52-cm-long strip heaters were 
used. In phase three, the instruments 
used to measure smoke characteristics, 
the CNM and the TEOM, were malfunction- 
ing, and the data could not be used. 



CALCULATIONS 



It is necessary to measure certain 
parameters in order to compare the 
steady-state combustion products and 
ultimately the hazards of various fuels. 
Among these measurements are gas concen- 
trations, smoke particle mass and number 
concentrations, ventilation rate, and 
mass-loss rate. Other combustion prop- 
erties can be calculated once these 
values are known. 

PRODUCT GENERATION RATES 

In a ventilated system, the generation 
rates (Gx) of CO2 and CO are related to 
the bulk average concentration increases 



above ambient, ACO2 and ACO, by the 
expressions 



Gco 2 ■ MC0 2 x v oAo * AC0 2 (1) 



and Geo = Mco x VqAq x ACO, 



(2) 



where Mx = density of the given gas, 
g/(m 3 *ppm), 

M C o 2 = 1.97 x 10" 3 g/(m 3 -ppm), 

Mco = 1.25 x 10 -3 g/(m 3 -ppm), 

and V A = incoming coal airflow, 
m 3 /s. 



COMBUSTION YIELDS 

Once the generation rates are known and 
the mass-loss rate of the fuel (Mf) is 
calculated using the load-cell assembly, 
the true yield of the combustion product 
(Y x ) can be calculated by the expression 



Y X = Gx/M f . 



(3) 



The yields for mass (M ) and number 
(N ) concentrations are calculated in a 
similar manner by the expression 



Y X = AX (C X )(VoA )/Mf, 



(4) 



Kx = stoichiometric yield of the 
given gas, g/g, 

Kco 2 = stiochiometric yield of CO2 
(2.86 g/g), 

and Kqq = stoichiometric yield of CO 
(1.82 g/g). 

Substituting the values in equations 1, 
2, and 5 yields 



Qa = VoA o [0.0214(AC0 2 ) 

+ 8.5 x 10" 3 (AC0)]. 



(6) 



where Cx = appropriate units conversion 
factor: 

1.00 x 10" 3 when M Q is in 
milligram per cubic meter 
or 

1.00 x 10 6 when N is in 
particle per cubic 
centimeter ; 

and AX = smoke concentration increase 
above ambient (when M is 
measured in milligram per 
cubic meter and N Q is mea- 
sured in particle per cubic 
centimeter) . 

HEAT-RELEASE RATES 

It has been shown (8) that the actual 
heat-release rate realized during a fire 
can be calculated from the expression 



5a ■ (& &c °-- 



Hq - Hep (Kco) 
KCO 



Geo, 



(5) 



where Qa = actual heat release, kW, 



He = net heat of complete combus- 
tion of the coal (31.0 
kJ/g), 

Hco = heat of combustion of CO 
(10.1 kJ/g), 



Since measurements of VqAo, ACO2, and ACO 
were made continuously, the actual heat- 
release rates could be calculated using 
equation 6. 

A typical fire rarely realizes the 
state of complete combustion. For this 
reason, the actual heat of combustion 
(Ha) during a fire is usually less than 
the net heat of combustion (He). By mea- 
suring both the actual heat-release rate 
(equation 6) and the fuel mass-loss rate, 
(Mf), the actual heat of combustion can 
be calculated from the expression 



H A = Q A /M f . 



PRODUCTION CONSTANTS 



(7) 



In an actual mine fire, it is often 
difficult, if not impossible, to calcu- 
late the actual heat of combustion. 
Moreover, since the true yield of a com- 
bustion product depends upon this infor- 
mation, significant errors can result in 
predicting the resultant concentration 
increases. For flaming fires, the rela- 
tive hazards tend to increase with the 
actual heat-release rate that results. 
For this reason, production constants, or 
beta values (8x), can be calculated for a 
given product by the expression 



Bx = G x /Qa. 



(8) 



Using the rate of formation of gas or 

smoke as a function of the fire size is 

also beneficial in comparing the combus- 
tion hazards of different fuels. 



SMOKE PARTICLE DIAMETERS 

Measurements of both number and mass 
concentrations of the smoke provide im- 
portant information relative to the 
yields (equation 4) and production con- 
stants (equation 7). They can also be 
used to calculate the average size of the 
smoke particles, using the expression 



ird, 



(Pp)(N Q ) = 1 x 10 3 M< 



(9) 



where 



Pp = individual particle den- 
sity, g/cm , 

d m = diameter of average mass, 
Mm, 



and 1 x 10 3 = the appropriate units con- 
version factor. 

Assuming a value of p p = 1.4 g/cm 3 , the 
diameter of average mass can be calcu- 
lated from 

1/3 

d m = 11.09 ( -^ ) > (10) 



■-(£) 



when the particle diameter is expressed 
in micrometers. 



Using the three-wavelength smoke detec- 
tor, the transmittance (T) of the light 
through the smoke can be calculated for 
each wavelength. The extinction- 
coefficient ratio can be calculated 
for each pair of wavelengths (A) 
from the following log-transmission 
ratios: 



lnT(Xl.OO) lnT(Al.OO) 



lnT(A0.63) lnT(A0.45) 



or 



lnT(A0.63) 
lnT(A0.45) 



Using these extinction coefficients and 
the curve in reference (_7) figure 11, the 
surface mean particle size (d32) can be 
determined. (Calculation of the 
extinction-coefficient curves assumes 
spherical particles with an estimated 
refractive index). 

The smoke obscuration is the percentage 
of light absorbed by the smoke or 100% of 
the light minus the percent transmission. 
It is calculated using the following 
equation: 



Obscuration = 100(1 - T) 



(ID 



The obscuration percentages presented 
below are an average of those calculated 
from the three wavelengths. 



COAL COMBUSTION RESULTS AND DISCUSSION 



All values listed in this report are 
averages for the steady-state burning 
stage, which was arbitrarily selected to 
be a 10-min period beginning 27 min after 
ignition. Figure 2 shows the results 
of a typical test. Ignition of the 
coal occurred at 23 min. The strip 
heaters were turned off at 43 min. The 
flames were quenched at 60 min. In 
this experiment, the steady-state 



values were obtained from 49 to 59 
min. 

Table 1 lists the ignition times and 
mass-loss data. At the higher ventila- 
tion rate, the coal ignited faster and 
burned more efficiently, as evidenced by 
the larger mass loss. Increasing the 
length of the strip heaters had about the 
same effect as raising the ventilation 
rate. 



TABLE 1. - Ventilation, ignition, and mass loss for 
the three phases of the coal experiments 





VoAo, 


Smoke 


Ignition 


Mass-loss 


Total mass 


Test 


m 3 /s 


sighted, 


sighted, 


rate, 


loss, 






min 


min 


g/s 


g 


1-6.. 


0.24 


12.0 


21.2 


0.39 


187 


7-8.. 


.44 


12.0 


18.0 


.45 


247 


9-21. 


.23 


11.5 


18.2 


.44 


231 




UJ 
CO 

< 

UJ 

_l 

UJ 

or 

UJ 

X 



1 u 1 1 1 1 1 1 1 
KEY 

~ Heat loss n / 

Mass loss / 1 / 


9 


8 

7 


/ >^A 


6 


" / / 




5 

4 


'-B J J 


L^-J— ^^-l— ; — -— . 


3 
2 

1 

o 



KEY 

Mass 

Number 





E 




3 


o> 




g 


or 

UJ 


» 


h- 


CO 
CO 

o 


Ul 

5 


_l 


< 


CO 


Q 


CO 


Ul 


< 


_l 


2> 


CJ 




1- 




cc 




< 




0_ 



-2.0 

-1.8 
ro 

Hl-6 .o 



10 20 30 40 50 60 70 80 90 




TIME, min 



80 90 



FIGURE 2.— Results of a typical coal combustion test. A, CO and C0 2 concentrations; B, particle mass and number 
concentrations; C, heat-release and mass-loss rates; D, diameter of average mass (d m ) and mean particle diameter (d 32 ). 



GAS CONCENTRATIONS AND HEAT PRODUCTION 

Figure 2A shows the typical gas produc- 
tion plot. The highest concentrations 
were produced while the strip heaters 
were on. Subsequently, the concentra- 
tions reached the steady state and re- 
mained there until the fire was quenched, 
which produced the final CO spike. 

Table 2 presents the CO and C0 2 con- 
centrations and generation rates and 
the corresponding heat-release rates 
and heats of combustion. At the higher 



ventilation rate, the gas concentrations 
were reduced owing to a greater dilution 
with incoming air. However, the genera- 
tion rates increased. This increase was 
more evident for CO2 because with an in- 
creased oxygen supply, the rapidly burn- 
ing coal was better able to support com- 
plete combustion. Increased oxygen was 
also responsible for a hotter fire, as 
can be seen from the greater heat-release 
rate. Using a larger ignition source 
produced a still hotter fire that doubled 
the gas production. These increased 



concentrations may have been the result 
of the strip heaters being in direct con- 
tact with more coal. 

Figure 25 shows how the heat-release 
rate corresponded to the mass loss. The 
mass loss rate dipped slightly after the 
strip heaters were turned off. 

HCN concentrations were also measured 
throughout experiments 1 through 6. The 
steady-state average concentration was 
6.5±1.5 ppm HCN. This is an indication 
of other potentially toxic gases, which 
are produced in trace amounts. At 
the same time, hydrogen sulfide was not 
detected (using the Draeger tube method). 

SMOKE CHARACTERISTICS 

Figure 1C shows the typical mass and 
number concentrations. The initial visu- 
al observation of smoke was confirmed by 
the number concentration, which showed an 
increase at 11 min. The concentration 
decreased while the coal was rapidly 
burning and increased again after the 
strip heaters were turned off. The mass 
concentration, however, showed a marked 
increase while the strip heaters were on 
and decreased when they were turned off. 

The smoke characteristics data are pre- 
sented in table 3. With increased ven- 
tilation, the burning coal seemed to 
produce more, but lighter, smoke parti- 
cles. Therefore, the calculated particle 



size was smaller. However, considering 
the broad diameter range at the lower 
ventilation rate, the size difference may 
be insignificant. When using the three- 
wavelength light transmission technique 
to calculate the particle size, the di- 
ameters seemed to overlap. Table 4 lists 
the diameters and smoke obscurations 
calculated from the three wavelength 
smoke detector data. Since all the 
diameters were in the same range, there 
may not have been an actual difference 
when comparing either the method of cal- 
culation or the effects of the ventila- 
tion rate. However, the larger ignition 
source seemed to produce a smokier fire, 
as indicated by the higher obscuration 
percentage. Figure 2D shows the par- 
ticle diameters as calculated by both 
methods. 

COMBUSTION YIELDS 

At the increased ventilation rate, all 
combustion yields were higher, especially 
the number and mass concentrations. The 
additional air supply caused a more pro- 
ductive fire; i.e., more gas and smoke 
was produced per gram of coal consumed. 
Using a larger ignition source caused an 
additional increase in the yield of 
CO and doubled the yield of CO2. The 
average yields are listed in table 
5. 



TABLE 2. - Gas concentrations, generation rates, heat-release 
rates, and heats of combustion for the coal experiments 



Test 


CO, ppm 


CO2, Ppm 


Geo, 
10' 2 g/s 


Gco 2 > g/ s 


Qa, kW 


Ha, kJ/g 


1-6 


89 

52 
176 


1,095 

834 

2,321 


2.7 
2.9 
5.1 


0.5 

.7 

1.1 


5.9 

8.0 

11.9 


15.2 
17.8 
27.1 



TABLE 3. - Smoke characteristics for the coal 
experiments 





N , 
10 6 p/cm 3 


M , mg/m 3 


d m 


Test 


Average, 


Range , 








um 


um 




1.5 


11.4 


0.21 


0.12-0.34 


7-8 


2.1 


8.1 


.17 


.15- .20 


9-21 


ND 


ND 


ND 


ND 



ND Not determined. 



10 



TABLE 4. - Mean particle sizes and smoke obscuration for the coal experiments 





lnT(Xl.OO) 


lnT(Xl.OO) 


lnT(X0.63) 


Average 

d32, 
ym 




Test 


lnT(X0.63) 


lnT(X0.45) 


lnT(X0.45) 


Obscuration, 




Average, 
ym 


Range, 
ym 


Average, 
ym 


Range, 
ym 


Average, 
ym 


Range, 

ym 


% 


1-6... 
7-8... 
9-21.. 


0.24 
.27 
.25 


0.17-0.31 
.16- .39 
.21- .30 


0.22 
.26 
.22 


0.16-0.29 
.19- .32 
.16- .29 


0.22 
.22 
.23 


0.18-0.26 
.18- .25 
.16- .36 


0.23 
.25 
.24 


20 
19 
29 



TABLE 5. - Combustion yields for coal experiments 



Test 


Yco, 10- 2 
g/g 


Yco 2 > g/g 


Y N , 10 11 

p/g 


Y Mq , 10" 3 
g/g 


1-6 

7-8 

9-21 


7.8 
11.6 
12.7 


1.3 
2.0 
2.6 


9.2 

34.8 

ND 


8.2 
18.3 

ND 



ND Not determined. 



PRODUCTION CONSTANTS 

Table 6 lists the production constants 
or beta values. These constants calcu- 
lated as a function of the fire size. 
The values for CO and CO2 remain fairly 



constant for all tests. However, at the 
higher ventilation rate, more inten- 
sive smoke production was reflected 
by the increased number and mass 
concentrations . 



FUEL COMPARISON RESULTS AND DISCUSSION 



The different combustion kinetics of 
coal and other fuels previously studied 
necessitated a modification of the ex- 
perimental conditions, but a comparison 
of their combustion products was still 
possible. Earlier studies of wood and 
transformer fluid fires were conducted in 
the same intermediate-scale fire tunnel 



using the same instrumentation and cali- 
bration techniques. However, the venti- 
lation rates and ignition sources varied 
with the fuel (table 7). 

The gas, heat, and smoke concentrations 
for the three fuels studied are found in 
table 8. The largest fire was produced 
by wood. (Heat release was used to 



TABLE 6. - Production constants for coal experiments 



Test 


6co, 10" 3 
g/kJ 


6co 2 , 10" 2 
g/kJ 


Bn , 10 10 
P/kJ 


6 Mo , 10- 4 
g/kJ 


1-6 


4.8 
4.5 
4.3 


8.9 
8.9 
8.9 


6.8 

13.7 

ND 


4.7 


7-8 


6.6 


9-21 


ND 



ND Not determined. 



TABLE 7. - Ignition source and ventilation rates 
for the three fuels tested 



Fuel 


Ignition source 


VqAo, m 3 /s 


Coal 


Electric strip heaters... 


1.0 
.24 


Transformer fluid... 


.47 



11 



determine fire size. ) The configuration 
of the wood sticks may have improved 
the air circulation, which could have 
supported more complete combustion than 
was achieved using the other fuels. For 
better comparison, the other fuels were 
normalized to the heat-release rate pro- 
duced by wood fires. Table 9 shows these 
normalized values. At the projected fire 
intensity, burning coal produced the most 
smoke and higher gas concentrations. 
However, burning transformer fluid pro- 
duced a thick, dense smoke. Of the fuels 
tested, the transformer fluid generated 
the largest particles and obscured the 
most light. These values are listed in 
table 10. 

Burning coal generated the most CO and 
the highest number of smoke particles, 



using the rate of formation of gas or 
smoke as a function of the heat produced. 
The production constants are found in 
table 11, and are confirmed by the con- 
centrations found in table 8, in which 
wood generated the most CO2 relative to 
the fire size. 

From these experiments, it would be 
misleading to say one fuel is more haz- 
ardous than the other two, since each one 
generates dangerous combustion products. 
In any mine fire, all the burning ma- 
terials combine to produce a wide variety 
of smoke particles and volatile gases 
that may be transported by the ventilat- 
ing system. If more efficient smoke de- 
tectors are to be developed, the indi- 
vidual components of the smoke should be 
known. 



TABLE 8. - Gas, heat, and smoke concentrations 
for the three fuels tested 



Fuel 



Wood 

Coal 

Transformer fluid 



CO, ppm 



145 

89 

113 



CO2, ppm 



6,759 
1,095 
1,769 



Qa, kW 



110.2 

5.9 

21.1 



N , 10' 
p/c 



rrr 



6.0 
1.5 
1.1 



M , mg/m 

4971 
11.4 
35.3 



TABLE 9. - Normalized gas and smoke concentrations 
for the three fuels 



Fuel 



Wood , 

Coal , 

Transformer fluid. 



CO, ppm 



145 

1,662 

590 



CO- 



ppm 



6,759 

20,452 

9,239 



N 



o, 10' 
p/cm 3 



6.0 
28.0 

5.7 



M , mg/m 3 



49.1 
213.3 
184.4 



TABLE 10. - Particle size and smoke obscuration 
for the three fuels tested 



Fuel 

Wood 

Coal 

Transformer fluid. . 
ND Not determined. 



d m , Pm 



d 3 2 , ym 



0.22 


ND 


.21 


0.23 


.39 


.39 



Obscuration, % 



9 
20 
46 



TABLE 11. - Production constants for the three 
fuels tested 



Fuel 


Bco, 10" 3 
g/kJ 


6co 2 , 10~ 2 
g/kJ 


Bn q , 10 10 
p/kJ 


$M o , 10" 4 
g/kJ 




1.6 
4.8 
3.1 


10.4 
8.9 
7.7 


5.8 
6.8 
2.5 


4.9 
4.7 
7.8 


Coal 


Transformer fluid. . 



12 



REFERENCES 



1. Egan, M. R. , and C. D. Litton. 
Wood Crib Fires in a Ventilated Tunnel. 
BuMines RI 9045, 1986, 18 pp. 

2. Egan, M. R. Transformer Fluid 
Fires in a Ventilated Tunnel. BuMines 
IC 9117, 1986, 13 pp. 

3. Lee, C. K. , R. F. Chaiken, J. M. 
Singer, and M. E. Harris. Behavior of 
Wood Fires in Model Tunnels Under Forced 
Ventilation Flow. BuMines RI 8450, 1980, 
58 pp. 

4. Smith, A. C, and C. P. Lazzara. 
Spontaneous Combustion Studies of U.S. 
Coals. BuMines RI 9079, 1987, 28 pp. 

5. McCaffrey, B. J., and G. Heskestad. 
A Robust Bidirectional Low-Velocity 



Probe for Flame and Fire Application. 
Combust, and Flame, v. 26, No. 1, 1976, 
pp. 125-127. 

6. Patashnick, H. , and G. Rupprecht. 
Microweighing Goes On -Line in Real Time. 
Res. and Development, v. 28, No. 6, 1986, 
pp. 74-78. 

7. Cashdollar, K. L. , C. K. Lee, and 
J. M. Singer. Three-Wavelength Light 
Transmission Technique to Measure 
Smoke Particle Size and Concentration. 
Appl. Optics, v. 18, No. 11, 1979, 
pp. 1763-1769. 

8. Tewarson, A. Heat Release Rate 
in Fires. Fire and Mater, v. 4, No. 4, 
1980, pp. 185-191. 



13 

APPENDIX. —SYMBOLS USED IN THIS REPORT 

Cx conversion factor of a given combustion product 

dm diameter of a particle of average mass, urn 

d32 mean particle size, ym 

Geo generation rate of CO, g/s 

Gco 2 generation rate of CO2, g/s 

Gx generation rate of a given combustion product, g/s 

Ha actual heat of combustion, kJ/g 

He net heat of combustion of fuel, kJ/g 

Hco heat of combustion of CO, kJ/g 

Kco stoichiometric yield of CO, g/g 

Kco 2 stoichiometric yield of CO2, g/g 

Kx stoichiometric yield of the given gas, g/g 

In logarithm, natural 

Mco density of CO, g/(m 3 «ppm) 

Mco 2 density of C0 2 . g/(m 3 *ppm) 

M f fuel mass loss rate, g/s 

M particle mass concentration, rag/cm 3 

M x density of the given gas, g/(m 3 *ppm) 

N particle number concentration, p/cm 3 

p particle 

Qa actual heat-release rate, kW 

T transmission of light, V 

V0A.0 ventilation rate, m 3 /s 

Yx yield of a given combustion product, g/g or p/g 

beta value (production constant) of a given combustion product, g/kJ or p/kJ 
measured change in a given quantity 

* wavelength, ym 

Pp individual particle density, g/cm 3 



3x 
AX 



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